Process of recovering alkalis from cement-kilns.



SPENCER B. NEWBERRY, OF BAYBRIDGE, OHIO.

No Drawing.

potash and soda, and that these alkalis are largely expelled in thecalcination of cement mixtures to clinker. It is also known that theflue-dust deposited from the gases escaping from the kilns contains ahigher proportion of alkalis than the raw mixture, and the possibilityof -recovering these alkalis from the flue-dust has often beendiscussed.

A long study of the expulsion of alkalis from"cement kilns and analysisof a great number of samples of mix, clinker, flue dust and stack gaseshave enabled me to establish the following facts: The amount of alkaliscontained in the clinker, under given conditions of burning, ispractically constant, and is not increased by increasing the proportionof alkalis in the raw mixture.

clinker can be greatly reduced, nearly to complete expulsion, byincreasing the temperature of burning and time of exposure to high heatbeyond the temperature and time ordinarily employed. Only a very smallpart of the alkalis volatilized in the The amount of alkalis remainingin the kiln is deposited with the flue-dust, and this is contained inthe flue-dust chiefly in a form insoluble in water. The remaining alkalican be almost completely recovered from the stack-gases by bringing thegases into intimate contact with finely divided sprays of water, or bypassing the gases through a space fitted with. porous material, thesurfaces of which are kept wet with water In this way is obtained asolution containing the smaller part of the alkali, in soluble form, andan insoluble sediment or -mud', containing the larger part of thealkali, in'insolul le form. If this insoluble matter is separated fromthe solution by filtering or siibsidence, and is returned to the kilnwith the fresh raw material, the percentage of alkali in the material isthereby increased, and since the alkali 1n the clinker remainspractically constant, 1t fol- Specification of Letters Patent. P t tedAug. 1?, 1915. -Application filed March 14, 1913. Serial No. 754,313. v

lows that an increased amount of'alkali is volatilized, and that astronger solution and a mud richer in alkali are obtained. Theproportion of soluble and insoluble alkali recovered remainspracticallythe same, and in consequence, by continually returning to thekiln the flue-dust and mud containing insoluble alkali, a constantlyincreasing amount of soluble alkali is recovered, until, after a time,an amount of soluble alkali practically equal to the total alkalicontained in the raw mixture supplied to the kiln is regularly obtained.For example, 600 lbs. of a dry raw mixture is required to produce 380lbs. of cement clinker, corresponding to one barrel of finished cement.The raw mixture contains 1.33 per cent. potasl1,,K O, or 8 lbs. perbarrel, and the clinker 0.70 per cent, or 2.66 lbs. per barrel. 10 lbs.of flue-dust are deposited for each barrel of clinker obtained, and thiscontains 3.4 per cent. potash, or 0.34 lbs. per barrel. The amount ofpotash escaping with the stack-gases will therefore be as follows:

In 600 lbs. mix 1.33 per cent 8. 00 lbs. In 380 lbs. clinker 2.66 In 10lbs. flue-dust 0.3a 3. 001bs.

Total in stack gases 5.00 lbs.

Soluble potash, in solution in water 2lbs. Insoluble potash, in 10 lbs.mud 3lbs.

If this mud and the fluedust are now returned to the kiln with the freshsupply of raw material, the latter: will now contain 8+0.34+3:11.34 lbs.potash, per barrel, which will yield clinker containing 2160 lbs.,flue-dust containing 0.34: lbs, and gases carrying 8.3a potash. Of thelatter, approximately two-fifths, or 3.34 lbs, will be recovered insoluble form, and 5 lbs. in the form of insoluble mud. Returning thelatter again to the, kiln, a further increased amount of soluble andinsoluble potash will be ob tained from the gases, until finally thegases will. carry "approximately 12.5 lbs. potash per barrel, of'which 5lbs. will be soluble, an amount equal to the total potash driven offfrom the fresh raw. mixture',-fand thereafter the composition offthe'gases and the amount of soluble and insoluble potash recovered from themwillbe substantially constant. j

In the above example, I have considered only the potash, as this is ofmuch greater value than soda, and is generally present in of alkalis,and that di erent cement materials will be found to vary considerably inpercentage of alkalis contained and expelled, and in the roportion ofthe latter deposited with the ue-dust and recoverable, respectively, insoluble and insoluble form. The chemical combination in which thesoluble alkalis are recovered differs according to the acid formingsubstances present in the mix ture and fuel. With ordinary materials,containing one per cent. or less of alkalis, the latter are generallyrecovered in solution as sulfate and chlorid, along with a considerableproportion of sulfate and chlorid of calcium and magnesium. Frommixtures rich in alkalis, however, such as are obtained by usingfeldspar or mica in place of clay or shale, and with fuels low insulfur, a large part of the soluble alkalis are recovered in the form ofcarbonate.

An important result of my experiments has been the discovery that theamount of alkalis retained by the clinker varies greatly with thetemperature and duration of the burning, and may be reduced toa minutepercentage, not exceeding one-tenth to twotenths per cent., bymaintaining in the hottest zone of the kiln a temperature somewhat hiher than that usually employed, but not su ciently high to fuse theclinker or injure the resulting cement, or by holding the clinkersomewhat longer than usual in the zone of hi hest heat. The latter caneasily be accomp ished by means of a ring of firebrick inside the liningatthe discharge end of the kiln, which has the effect of holding agreatly increased Volume of clinker in the hot zone, and. thus retardingthe rate of travel of the clinker through this part of the kiln.

Another device which I have found of great advantage in effecting therecovery of the maximum amount of alkalis is to cool the gases from thekiln, after they have been ther quantity of flue-dust and cooled to a.

point at which they can be effectively treated for alkali-recovery bywashing with a small volume'of water. The heat conducting surface mayalso consist of the tubes of economizers for heating boiler-feed water,or the tubes of air-heating coils for heating the airblast .of the kiln.

-, After leaving the heat absorbing surfaces,

the gases may with advantage be conducted under pans in which thesolution of alkalimetal salts, obtainedby washing the gases, isevaporated to dryness. The gases then pass to the absorbing apparatus,preferably consisting of-a tower, the lower part of which consists of areservoir for solution, above which is an empty section provided with anumber of water sprays, in which the gases are brought into intimatecontact with water or alkali-metal salt solution-in a very finelydivided condition and are thus effectively freed from suspendedinsoluble and soluble matter. Above the spray-chamher is a section ofthe tower filled with a loose checker-work of brick or loosely-piledporous material such as coke, the surfaces of which-may be kept'wet bymeans of an ad ditional water spray at the top of the tower. The efi'ectof this device is to free the gases to a great extent fromsuspendedmmute drops of solution in the form of a 'fog, by collectingthese into larger drops and streams and thus preventing their escapewith the gases discharged from the tower. At the top of the absorbingapparatus is placed an exhaust fan, the capacity and speed of which areso adjusted as to remove and dischargethe treated gases. and to maintaina suitable draft in the kiln.

The sprays are preferably supplied with alkali-metal salt solution,drawn by a pump from the reservoir at the base of the tower, to whichwater is added as required to make up for that evaporate-d by the heatofthe gases. In this way. the solution becomes concentrated by absorptionof further portions of alkalis, and is collected, with a considerableamount of insoluble mud, in the reservoir. A portion of the solution iscontinuously or from time to time removed and. evaporated to dryness orto the point of:

crystallization of alkali-metal salts, in the pans above mentioned. Themud settlesrapidly from the solution, and may be removed at intervals bysuitable devices, or the liquid and mud may be drawn from the tower intoseparate reservoirs, and after settling or filtering, the clarifiedliquid is returned to the tower. ably also the flue-dust are returned tothe v kiln with the fresh raw material introduced.

By operating substantially as above described'I am able to recoverpractically all the alkalis, hitherto lost fromcement kilns with thestack-gases, in the form of watersoluble salts, containing from 30 to 50per cent. of alkalis, chiefly potash, and thus to obtain a by-product ofcement manufacture, in extensive quantities, which is of great value inagriculture and the chemical arts.

Having thus described the invention, What is claimed is:

The process of recovering alkalis from the The mud, and preferastack-gases of cement kilns, which consists in S9p2IfltlIlgflU8-dllStfrom the gases, coolmg the gases by contact with hea -absorb1ng surfacesWashin the ases with a ueous liquid, freeing the washed gases from suspended liquid by passing them through a porous layer, separatinginsoluble matter from the solution of alkali-metal salts s0 obtained,returning-the flue-du and insoluble matter to the kiln, evaporating partof the solution and returning the remaining solution to the liquid usedfor Washing the gases.

In testimony whereof I aifix my signature in presence of two Witnesses.

SPENCER B. NEWBERRY.

Witnesses HAT'rna M. llfinesrrr, HARVEY N. BARRETT.

